The present invention concerns centrifugal casting machines of the broken arm type in general and casting machines employed to cast dental prostheses in particular. However, the present invention is not necessarily limited thereto and it concerns centrifugal casting machines which may be used for other purposes such as the casting of jewelry and other items.
Generally, such centrifugal casting machines include a base on which a casting arm is mounted for rotation, usually in a horizontal plane, by an electric motor or suitable other means. The so-called "broken arm" type of centrifugal casting machine has a swing arm pivotably mounted on the casting arm. The swing arm has means thereon to support a crucible and a casting ring in which a suitable mold is carried. The crucible contains a molten metal, which is usually melted in place in the crucible by a blowtorch or other means. When the metal is melted, the casting arm is then rotated, and rotation forces and the inertia of the molten metal cooperate to force the metal from the crucible into the mold. The swing arm is initially mounted at an angle to the casting arm to better align the crucible and mold during start up, and the swing arm pivots outwardly into alignment with the casting arm as the latter picks up speed.
One problem with prior art broken arm centrifugal casting machines is that upon initial rotation of the casting arm there is a tendency for the molten metal to be diverted from its true path into the mold and to splash or travel up the side walls of the crucible with attendant loss and poor casting results. Generally, it is desired that the molten metal enter the mold as quickly and completely as possible.
One prior art device which addresses this problem is shown in U.S. Pat. No. 2,297,973 entitled CENTRIFUGAL CASTING MACHINE issued on Oct. 6, 1942. This patent shows a centrifugal casting machine having a pivotable swing arm initially disposed at an acute angle to the casting arm and having a cam follower thereon. The base of the machine has a cam which engages the cam follower and is configured to accelerate outward pivoting movement of the swing arm during the start up or initial period of rotation to better align the mold with the crucible to reduce splashing of metal as the machine picks up speed. However, the teaching of this patent has a shortcoming in that as the swing arm pivots, its orientation to the path of rotation of the casting arm is constantly changing, and at a deliberately accelerated rate. Therefore, the time interval during which the swing arm might be properly oriented to the metal path of flow is actually reduced. The teaching of the patent is devoid of any recognition of the concept of orienting the swing arm to the metal flow path during start up and merely attempts to make a "running catch" of the molten metal. The difficulty is that a given cam configuration provides a given acceleration dependent upon the speed of rotation of the casting arm, and differences in metal mass and crucible configuration for different casting operations are not accounted for.
Other patents showing various casting devices are U.S. Pat. Nos. 1,466,826; 1,527,729; 2,125,080 and 2,192,043.
Another problem with prior art centrifugal casting devices is the necessity for the metal to travel up a wall of the crucible from the recessed holding portion of the crucible to its pouring lip. The molten metal must thus overcome frictional resistant forces in addition to its own inertial resistance to movement, and the likelihood of splashing or diversion of the metal from the desired true flow path into the mold is increased.